In a synchronous motor (permanent magnet synchronous motor), which has become widely used to drive a compressor and the like, control in which the position of a rotor (rotation angle of the rotor) is sensorlessly detected to appropriately energize stator coils, is performed. The synchronous motor that is normally operated in such a sensorless mode by position detection operation needs to be in a start mode involving forced commutation operation at the time of start, as a stage preceding the sensorless mode. In the start mode, forced commutation of a drive signal is performed without rotor position detection, and the rotor is forced to rotate regardless of its position (for example, Patent Document 1).